Light crude oils have long been favored by refiners due to their ease of refining and lack of needed additional treatment. However, as demand for crude oil increases and the availability of light crude declines, refiners have turned to heavier crude oils for refining. This requires development of methods and equipment for producing, handling, transporting, and refining medium, heavy, and extra heavy crude oil.
Venezuela and Canada produce much of the heavy crude or bitumen, which has a gravity of 6 to 9 API. Diluents are typically used in the production of this heavy crude to raise the gravity to 16 to 18 API. One of these production methods is known as Steam-Assisted Gravity Drain (SAGD). The SAGD process operates by injecting steam into the formation at temperatures of up to 600° F. and pressures up to 1200 PSI.
As the bitumen is brought to the surface, the temperature drops to 400-440° F. The bitumen contains 70% water, but even at 440° F. it remains heavier than water and therefore sinks. In order to use traditional oil production equipment, the API of the bitumen is raised to about 16 API by the addition of diluents equivalent to 15-35% by weight. This results in a temperature drop of the emulsion to about 300° F. with about 50-55% water cut. The resulting diluent emulsion (commonly referred to as dilbit) is fed to a free water knockout tank (FWKO) where most of the water is removed by gravity. From the FWKO the emulsion containing 10-25% water is fed into an electrostatic treater where it is electrostatically dehydrated.
Another production method for extra heavy oil is surface mining operations. Tar sands dug from the earth are transported by conveyor belt to a ball crusher for size reduction. The crushed ore is fed to a steam stripper at 185° F. and slurried with hot water and caustic soda. The froth is then pumped to a Primary Separation Vessel (PSV) where the froth rises to the surface of the vessel and a diluent is then added. The emulsion is then fed into a centrifuge for separation of water and other solid particles.
In certain applications flash treaters are used to remove water from the dilbit. The 300° F. bitumen is depressurized into a vessel which permits the water to be removed as vapor. However, flash dehydration leaves crystalline salt in the crude which causes severe desalting problems at the refinery.
The problem with the prior art method of processing heavy crude is the sheer volume of diluent needed. For example, the volume of bitumen reserves in Canada is estimated to be 1.7 trillion barrels. It is estimated that only 10% of this bitumen is recoverable using currently known technology. Whether the bitumen is produced by mining or SAGD it must be diluted for transport and processing. Assuming a modest diluent usage of only 15%, the industry will need 25 billion barrels of diluent to sustain bitumen production. Assuming this production lasts for 15 years, the diluent demand would be nearly 1.7 billion barrels per year. This is well beyond any rational expectation.